The invention described herein is a pressure transducer utilizing a magnet and Hall effect sensor to convert mechanical movement into electrical signals. The pressure transducer without a Hall effect sensor is well understood in the art. A pressure transducer typically includes two plenums or chambers separated by a flexible diaphragm. The plenums are subject to a fluid, such as a gas or a liquid, under different pressures. This pressure differential causes displacement of the diaphragm in the direction of the plenum exposed to the lower pressure. The diaphragm is connected to a mechanical component, which moves as the diaphragm is displaced.
For example, the mechanical component could be a piezoelectric strip having two ends, one end fixed to a plenum wall or other fixed point of reference and the other end fixed to the diaphragm, and thus moving with the diaphragm. The electrical properties of the piezoelectric strip change as it is bent by the movement of the diaphragm. The electrical change can then be processed by other electrical components to indicate the pressure value. Other devices to convert the mechanical movement into electric signals are also used to convert the movement of the diaphragm as recognized by one skilled in the art.
In the present invention, a magnet and Hall effect sensor are used to convert the mechanical movement to electric signals. It should be understood that the power conserving features of the invention disclosed herein need not be limited to the use of a Hall effect device, but may be applied to implementations using other structures for sensing movement as well. The power circuit can also be used in other applications where a low average current draw is required.
The use of a Hall effect sensor and a magnet allows for physical separation of the mechanical and electrical components of the pressure transducer, since the interaction between the magnet and the Hall effect sensor can take place with material between them. The preferred embodiment shows an arrangement where the fluid in the differential pressure sensor and the mechanical beam and magnet are separated from the electronics, including the Hall effect sensor, by a wall.
Many pressure transmitter applications use a 4 to 20 mA current loop to operate. In such instances, the 4 to 20 mA current loop provides both device power and signal path for the control signal. Most current loops used in the applications are specified to run between 10 and 35V supply voltage. As a result of the supply voltage and current constraints, the pressure transmitter should be capable of operation at a power level equal to the lowest power available, or 4 mA×10V=40 mWatts. In other words, because the power available can vary with the current signal, and from application to application, the maximum amount of power that can be relied upon is 40 m Watts. Thus, the device needs to be able to run on that amount of power, even if a higher amount is available in some applications.
A problem arises in that most Hall effect devices available today consume more power than 40 mW. For example, the HAL805 and HAL810 available from Micronas need at least 10 mA of current available at a minimum of 4.5V to work. This is a minimum power load of 10 mA×4.5V=45 mW to achieve operation. With such power requirements, not only is there no power left over to run the rest of the transmitter, there is not enough power to run the Hall effect sensor itself.
The problem is solved by alternately applying Power to the Hall sensor, making a measurement, then removing power for an extended time interval. In such a fashion, the duty cycle of the Hall sensor is adjusted. The position of the diaphragm or beam of the transducer does not need to be continuously monitored. In most applications, the Hall effect sensor can be turned off for substantial portions of the time, thus saving substantial amounts of power. In typical applications, the duty cycle of the sensor can be set anywhere from 1:2 to 1:100, although other ranges beyond those figures are possible. While other ranges are possible, the limitations of the average current draw and surge current to the sensor still must be achieved. In some embodiments, the duty cycle or period applying power can be selectively varied by the user. Such a selective circuit may include a multi-position switch, or other structure to allow a user to select from a plurality of timing circuits.
In the case of an integrated semiconductor sensor, such as the HAL810 from Micronas, the sensor can be turned on and a stabilized reading made in approximately 40 mSeconds. If a reading is taken once every 400 mSeconds, the average power requirement of the sensor can be reduced by a factor of ten. This means the average power draw of the Hall effect sensor is reduced to 4.5 mW for an average current draw from the sensor of only 1 mA at 4.5 volts. This is an amount of power that can be delivered to the sensor in a current loop situation. The remaining approximately 35 mW of available power can be used to run the rest of the transmitter.
Similarly acceptable duty cycles can be done with most other Hall effect sensors depending upon their power draw and start up and stabilization times. To achieve the power reduction, a suitable power supply is needed for the Hall effect sensor that can draw energy from the 4 to 20 mA loop on the average basis and deliver energy to the Hall effect sensor on the peak basis that the sensor requires. As described in the preferred embodiment, a low pass filter is utilized to draw power from the current loop and store the power until it is provided to power the Hall effect sensor. One skilled in the art will recognize that the power supply described herein is but one of many power supplies that can meet the goals described above.